This invention relates to apparatus for continuously producing photovoltaic devices on a substrate by depositing successive semiconductor layers in each of at least one deposition chamber through which the substrate continuously travels. In the preferred embodiment, the deposition of amorphous semiconductor layers is accomplished by glow discharge deposition techniques in which the process gases are decomposed and recombined into as yet not fully understood molecular species and compounds under the influence of an electromagnetic field. The composition of the amorphous semiconductor layers deposited onto the substrate is dependent upon, not only the particular process gases introduced into each of the deposition chambers, but also the particular molecular structure of the species and compounds at the time of deposition. It is therefore important to carefully control, not only the composition of the process gases introduced into the deposition chamber, but also the molecular structure of the semiconductor material deposited across the entire surface area of the substrate. To the end of controlling the composition of the process gases, the deposition apparatus is sealed from the atmosphere, pumped to low pressures, heated to high temperatures, flushed with a gas such as hydrogen, silane or argon prior to initiation of the glow discharge deposition process, and equipped with a precathode assembly for removing contaminants from walls of the chamber and impurities from the process gases and initiating the disassociation and recombination of the process gases. The concept of a precathode assembly has been fully disclosed in U.S. patent application Ser. No. 452,224 filed Dec. 12, 1982, entitled UPSTREAM CATHODE ASSEMBLY, and assigned to the assignee of the instant patent application. While the precathode assembly referred to above solved the problem of uniformity of deposited semiconductor layers in static or batch-type glow discharge deposition systems, lack of uniformity of the deposited semiconductor layers in deposition systems employing a continuously moving substrate was only partially improved. Although said patent application Ser. No. 452,224 includes an embodiment depicting process gas flow in a direction substantially parallel to the direction of travel of the web of substrate material (see FIG. 6 thereof), the disclosure therein neither mentions nor suggests (1) the use of said parallel process gas flow in combination with a profile gas to obtain the deposition of profiled semiconductor layers, or (2) providing said parallel process gas flow in a direction opposite to the direction of travel of the web of substrate material. It is to the end of further improving the uniformity of semiconductor layers deposited onto a continuously moving substrate that the present invention is directed. It is to the additional end of depositing profiled semiconductor layers that the instant invention is also directed.
In order to understand the problem of depositing semiconductor films exhibiting uniform optical, electrical and chemical characteristics, it is necessary to describe, in greater detail, the process gas introduction systems of prior art deposition apparatus for depositing semiconductor material, as well as to accentuate the differences between static or batch-type systems and continuous systems. In previous deposition systems, whether batch or continuous, the process gases were (1) introduced, at spaced intervals, along one transverse edge of the substrate; (2) drawn, by a vacuum pump, transversely across the deposition surface of substrate where a cathode or antenna energized by electromagnetic energy developed an electromagnetic field in the region defined between the deposition cathode or antenna and the substrate (hereinafter referred to generally as the "decomposition region" and specifically as the "plasma region"); (3), upon entering the electromagnetic field, disassociated and recombined into an ionized plasma made up of species and compounds of the originally introduced process gases; and (4) deposited onto the heated deposition surface of the substrate.
However, deposition systems which include a moving substrate, even when equipped with the previously described composition controlling devices such as precathode assemblies, have been found to deposit semiconductor material adjacent the upstream section of the substrate (that section of the substrate first contacted by the process gases) which exhibits different (nonuniform) optical, electrical and chemical characteristics than the semiconductor material deposited over a more downstream portion of the deposition surface of the substrate (that section of the substrate later contacted by the process gases). The differing optical, electrical and chemcial characteristics are believed to be primarily attributable to the introduction of process gases in a direction transverse to the direction of travel of the substrate. More particularly, and referring specifically to FIG. 3A, when process gases are introduced and made to flow transversely across the substrate 11, in the direction depicted by arrow A (the substrate 11 is moving in the direction depicted by arrow B), deposition of the semiconductor material begins at edge 11a of the substrate 11 and proceeds thereacross to opposite edge 11b. It is believed to be because (1) certain of the process gases introduced into the deposition chamber are deposited onto the substrate more rapidly than others, thereby partially depleting the downstream gas mixture, and (2) the chemical combinations and bonding formations of the process gases change with the length of time the process gases are subject to the effects of the electromagnetic field, that the optical, electrical and chemical properties of the deposited semiconductor layer change with a corresponding transverse change of position across the width of the substrate 11 (assuming that all other deposition parameters such as gas and substrate temperature are constant). Obviously, the direction of introduction of process gases as related to uniformity of deposited semiconductor material is irrelevant in batch systems, however, the direction of process gas introduction as related to uniformity of deposited semiconductor material becomes significant with systems which employ a moving substrate. Note that in the mass production of photovoltaic devices, a web of substrate material is continuously fed through the chambers for the deposition thereonto of semiconductor layers. If the deposited material at the downstream end is electrically, optically or chemically inferior, it may be severed from the deposited upstream material. While in this manner the overall efficiency of the photovoltaic devices is not harmed, the severed material represents considerable economic waste.
Accordingly, and referring to FIG. 3B, it is the principle object of the present invention to provide a deposition system wherein the process gases are introduced into the deposition chamber so as to flow across the substrate 11 in a direction, depicted by arrows C, which is substantially parallel to the direction of travel of the substrate 11, depicted by arrow B. Note that, while the process gases are illustrated as flowing in a direction parallel, but opposite to the direction of substrate movement, the gases could also flow parallel and in the same direction as the direction of substrate movement without departing from the spirit and scope of the present invention. In either event, it has been found that by introducing the process gases in a direction substantially parallel to the direction of travel of the substrate 11, the semiconductor material deposited thereonto, although "graded" (slices of the material taken in a direction parallel to the plane of the substrate vary in homogeneity with respect to one another) is substantially "uniform" (slices of material taken in a direction perpendicular to the plane of the substrate, which slices comprise an aggregation of the graded layers, exhibit substantially similar chemical, optical and electrical properties). This is believed to be due to the fact that all of the process gases adjacent any arbitrarily selected line which extends across the entire transverse width of the moving substrate 11 have been subjected to the effects of the electromagnetic field for substantially identical lengths of time. The species and compounds formed from those disassociated and recombined process gases, being at substantially identical stages of disassociation and recombination, are therefore deposited onto the entire surface of the substrate as a film of substantially uniform semiconductor material.
More particularly and still referring to FIG. 3B, imaginary line D--D has been arbitrarily selected to extend across the entire transverse width of the moving substrate 11 at a location downstream of the point of entry of the process gases, while line E--E has been arbitrarily selected to extend across the entire transverse width of the moving substrate 11 at a location upstream of line D--D. As the process gases flow parallel to the path of travel of the substrate 11, depicted by arrow B, and arrive at line D--D, certain ones of the species and compounds, formed from the disassociation and recombination of the process gases under the influence of the electromagnetic field are deposited onto the substrate 11. Since line D--D is adjacent the side of the substrate 11 first entering the deposition chamber (note again the direction B of substrate travel) and certain constituents of the process gases are inherently deposited more rapidly than others, it is likely that the more rapidly deposited constituents are at least partially depleted from the process gases arriving at line D--D, thereby resulting in a different composition of semiconductor material deposited onto the substrate adjacent line D--D than the semiconductor material deposited adjacent line E--E. Further, since the nondeposited process gases adjacent line E--E have not been subjected to the electromagnetic field for as long a period of time as those process gases adjacent line D--D, these process gases have not as yet been disassociated and recombined into substantially identical species and compounds, thereby also contributing to the deposition of differing compositions of semiconductor material. However, in contrast to a deposition system employing a transverse flow of process gases across the substrate 11 in which the optical, chemical and electrical properties of the deposited material along the width of the substrate would vary, the parallel process gas introduction and channeling system of the instant invention deposits uniform semiconductor material across the entire surface of the substrate.
As should now be clear, the uniformity of deposited material is provided because the process gases at any given point across the transverse width of the substrate (such as at any point along line D--D or at any point along line E--E) have been subjected to the electromagnetic field for substantially identical periods of time and, consequently, have disassociated and recombined into substantially identical species and compounds. Accordingly, a layer of semiconductor material formed from the species and compounds present in the plasma adjacent line D--D is continuously deposited atop the entire surface of the substate 11 adjacent the substrate entry side of the deposition chamber, and a layer of semiconductor material formed from the species and compounds present in the plasma adjacent line E--E is continuously deposited atop the entire surface of the semiconductor material previously deposited adjacent line D--D atop the entire surface of the substrate 11 adjacent the substrate exit side of the deposition chamber. The semiconductor material deposited onto the substrate 11 in any given deposition chamber is therefore an aggregation of the semiconductor material deposited adjacent an infinite number of hypothetically drawn lines (such as lines D--D and E--E), each line extending across the width of the substrate 11. This aggregation of layers, each of infinitely small cross-sectional thickness and substantially uniform and homogeneous chemical composition, will be referred to hereinafter as "graded" semiconductor layers. While the deposited semiconductor material might be nonuniform and vary in optical, electrical and/or chemical characteristics if a plurality of cross-sectional slices of that deposited material were taken in a direction parallel to the plane of the substrate and those slices were compared, the deposited semiconductor material would be substantially uniform, graded and exhibit similar optical, electrical and chemical characteristics if a plurality of cross-sectional slices were taken in a direction perpendicular to the plane of the substrate and compared.
Finally, note that in the perferred embodiment of the present invention, the deposition apparatus will be described with specific reference to glow discharge deposition apparatus. However, the process gas introducing and channeling system, presented herein, is applicable to any type of deposition system which employs a continously moving substrate onto which decomposed process gases are depositied, whether by glow discharge deposition techniques, chemical vapor deposition techniques, or heat assisted chemical vapor deposition techniques, etc.
Recently, considerable efforts have been made to develop processes for depositing amorphous semiconductor films, each of which can encompass relatively large areas, and which can be doped to form p-type and n-type materials for the production of p-i-n-type devices substantially equivalent to those produced by their crystalline counterparts.
It is now possible to prepare amorphous silicon semiconductor alloys, by glow discharge or vacuum deposition techniques, said alloys possessing (1) acceptable concentrations of localized states in the energy gaps thereof, and (2) high quality electronic properties. These techniques are fully described in U.S. Pat. No. 4,226,898, entitled Amorphous Semiconductor Equivalent to Crystalline Semiconductors, issued to Stanford R. Ovshinsky and Arun Madan on Oct. 7, l980; by vapor deposition as fully described in U.S. Pat. No. 4,217,374, issued to Stanford R.Ovshinsky and Masatsugu Izu, on Aug. 12, 1980, under the same title; and in U.S. patent application Ser. No. 423,424 entitled Method Of Making Amorphous Semiconductor Alloys And Devices Using Microwave Energy by Standord R. Ovshinsky, David D. Allred, Lee Walter and Stephen J. Hudgens. As disclosed in these patents, it is believed that fluorine introduced into the amorphous silicon semiconductor operates to substantially reduce the density of the localized states therein and facilitates the addition of other alloying materials, such as germanium.
The concept of utilizing multiple cells, to enhance photovoltaic device efficiency, was discussed at least as early as 1955 by E. D. Jackson, U.S. Pat. No. 2,949,498 issued Aug. 16, 1960. The multiple cell structures therein discussed utilized p-n junction crystalline semiconductor devices. Essentially the concept is directed to utilizing different band gap devices to more efficiently collect various portions of the solar spectrum and to increase open circuit voltage (Voc.). The tandem cell device has two or more cells with the light directed serially through each cell, with a large band gap material followed by a smaller band gap material to absorb the light passed through the first cell or layer. By substantially matching the generated currents from each cell, the overall open circuit voltage is the sum of the open circuit voltage of each cell while the short circuit current remains substantially constant.
Hamakawa et al, reported the feasibility of utilizing Si-H in a configuration which will be defined herein as a cascade type multiple cell. The cascade cell is hereinafter referred to as a multiple cell without a separation or insulating layer there between. Each of the cells was made of an Si-H material of the same band gap as in a p-i-n junction configuration. Matching of the short circuit current (J.sub.sc) was attempted by increasing the thickness of the cells in the serial light path. As expected, the overall device Voc increased and was proportional to the number of cells.
It is of obvious commercial importance to be able to mass produce photovoltaic devices such as solar cells. However, with crystalline cells mass production was limited to batch processing techniques by the inherent time consuming growth requirements of the crystals. Unlike crystalline silicon which is limited to batch processing for the manufacture of solar cells, amorphous silicon semiconductor alloys can be deposited in multiple layers over large area substrates to form solar cells in a high volume, continuous processing system. Continuous processing systems of this kind are disclosed, for example, in pending patent applications: Ser. No. 151,301, filed May 19, 1980 for A Method of Making P-Doped Silicon Films and Devices Made Therefrom; Ser. No. 244,386 filed Mar. 16, 1981 for Continuous Systems For Depositing Amorphous Semiconductor Material; Ser. No. 240,493 filed Mar. 16, 1981 for Continuous Amorphous Solar Cell Production System; Ser. No. 306,146 filed Sept. 28, 1981 for Multiple Chamber Deposition and Isolation system and Method; and Ser. No. 359,825 filed Mar. 19, 1982, for Method And Apparatus For Continuously Producing Tandem Amorphous Photovoltaic Cells. As disclosed in these applications, a substrate may be continuously advanced through a succession of deposition chambers, wherein each chamber is dedicated to the deposition of a specific material. In making a solar cell of p-i-n-type configuration, the first chamber is dedicated for depositing a p-type amorphous semiconductor material, the second chamber is dedicated for depositing an intrinsic amorphous semiconductor material, and the third chamber is dedicated for depositing an n-type amorphous semiconductor material.
It is for use with continuous deposition apparatus such as the systems described in the patent applications cited hereinabove that the process gas introduction and channeling system of the present invention is directed. When equipped with the instant introduction and channeling system, those deposition apparatus are adapted to deposit more uniform layers of semiconductor material than previously possible, and, consequently reduce waste and produce more efficient photovoltaic devices.
The further objects and advantages of the present invention will become clear from the drawings, the claims and the description of the preferred embodiment which follow.